bash: allow setting lsColorsFile to null

Signed-off-by: misilelab <[e-mail]>

nixos/modules/programs/bash/ls-colors.nix

···
       default = true;
     };
     programs.bash.lsColorsFile = lib.mkOption {
-      type = lib.types.path;
-      default = "";
+      type = lib.types.nullOr lib.types.path;
+      default = null;
       example = lib.literalExpression "\${pkgs.dircolors-solarized}/ansi-dark";
       description = "Alternative colorscheme for ls colors";
     };
···
 
   config = lib.mkIf enable {
     programs.bash.promptPluginInit = ''
-      eval "$(${pkgs.coreutils}/bin/dircolors -b ${config.programs.bash.lsColorsFile})"
+      eval "$(${pkgs.coreutils}/bin/dircolors -b ${
+        lib.optionalString (config.programs.bash.lsColorsFile != null) config.programs.bash.lsColorsFile
+      })"
     '';
   };
 }